A six-channel intraoral transmitter for measuring occlusal forces

A SIX-CIWNNEL INTRAORAL TRANSMITTER MEASURING OCCLUSAL FORCES

FOR

IAN SCOTT* AND M. M. ASH, JR., D.D.S., MS.“” University

U

of Michigan,

School oj Dentistry,

Ann Arbor,

Mich.

RECENTLY, tooth contacts during swallowing and mastication have been studied indirectly. Jankelson and coauthors1s2 appear to be the first to measure directly the occurrence of tooth contact by means of applying an electrical potential between two antagonistic full crowns. However, Brewer and Hudson” overcame the disadvantage of wires extending from the mouth by building a radio transmitter into an upper denture and telemetering the duration and frequency of tooth contacts. Later Neil14 also studied the contact between dentures using incorporated radio transmitters. It was after Brewer’s3 report that Graf,6 Gillings,G and Adams and Zander’ reduced the size of the transmitters so as to be placed in the pontics of removable partial dentures. These studies have greatly extended our knowledge of contact relations of the teeth and stimulated widespread interest in the use of radio transmitters for monitoring oral phenomena. Their research has pointed out the need for more precise and complex transmitters and the problems associated with the development of such devices. It is now apparent that sensors and telemetering equipment can be developed and used to monitor occlusal forces continuously during function and nonfunction (bruxism), swallowing, muscle forces, and jaw positions and movements. The development of sufficiently small and economically feasible transmitters for general use would greatly benefit all investigators interested in various aspects of the masticatory system. Under such conditions, many of the problems of studying sensory, muscle, and occlusal functions could be attacked effectively under a variety of conditions using statistically significant numbers of subjects. NTIL

TECHNICAL

PROBLEMS

The technical problems in the development of complex intraoral transmitters are difficult to resolve because of the dimensional and power requirements associated with the miniaturization. A major problem in the development of ultraminiature intraoral transmitters is the absence of a commercial demand for inte*Project Engineer. **Professor of Dentistry. Supported by United States Public General, Contract #DA-49-007.MD721,

Health

Grant

56

DE-01962, and U.S.A., ofece of Surgeon

Volume

16

Nujnber 1

SIX-CIIANNEL

INTRAORAL

TRANSMITTER

57

grated circuit packages of the size and type required. Their development within the limits of budget and technologic feasibility requires unique electronic equipment and personnel. The initial objective in the development of a system for the analysis of occlusal forces was the development of a six-channel transmitting and receiving and discriminating equipment, Since it was desired that occlusal forces be studied in opposing teeth simultaneously, it was necessary that transmitting, receiving, and decoding equipment be capable of twelve-channel operation. Six channels of information were desired for each molar pontic so that vertical, lateral, and rotational forces could be studied. At the present time, force sensors for vertical forces are placed in the mesial and distal fossae, and force sensors for lateral and rotational forces are placed in the mesial and distal cusps on both the buccal and lingual aspects of a pontic (Fig. I). Other locations of the force sensors may be made for additional force analyses. TRANSMITTER

In arriving at the final transmitter design, a great many systems have been considered and discarded on the grounds of practicability, physical size, cost, or dev’elopment time. The present transmitter design satisfies most of these considerations. Fig. 1 shows a block diagram of the transmitter. The transmitter allows six channels of information to be telemetered simultaneously. Six modulating codes or subcarriers on a single carrier are specified in the construction of the transmitters. The code associated with a specific transducer and area of tooth is identified by its frequency. Frequency may be that of a simple sine wave, or a more complex wave such as a repetitive pulse train in which pulse repetition rate may be considered synonymous with frequency.

,

Anatomical Tooth Sutfaee Metal Smror Form

-

Six-C*rmic

Copocitots

Antenna Coil Six-Transistor

Flat Pack

Resistor SubStmte Ceramic Capacitor

Fig. 1.-A buecolingual cross-section sensc~rs and other electronic components

of a molar pontic of the six-channel

illustrating transmitter.

the positions

of force

58

SCOTT

AND

ASH

J. Pros. Den. January-February, 1966

The transmitter may be visualized as radiating as a convenient frequency f,), and being modulated by six lower frequencies, f,, f,, f,, f,, f,, f,, each of which varies over a limited range with the application of occlusal forces or contacts in various areas of the occlusal surfaces of a molar pontic. Although the choice of f, is relatively uncritical, certain factors must be considered. The fixed size of the transmitter sets a limit to the physical configuration of the radiating antenna; radiated power will increase with frequency; and body attenuation increases with frequency. A range for f, has been set at 3 to 10 megacycles per second. This frequency range is above that of most highpowered interference (broadcast transmitters, electrical interference, etc.), is below any frequency at which body attenuation is likely to be significant, and is adequate to handle the required data rate. The exact value of f, is determined largely by practical considerations in the design of the transmitter. The choice of modulating frequencies is more complex. Minimum frequencies are set by the required data rate, and the relationship between frequencies must be such that cross modulation between channels is held to a minimum. It is essential that the individual modulation frequency bands do not overlap, and desirable that they do not have mutual harmonics within the total modulation spectrum. As indicated in Fig. 2, there are six solid state epitaxial force sensors. All of the upper unijunction bases are connected together and returned to B+ through a low impedance. Six trains of pulses, corresponding f,, f,, f, . . . f,, therefore, appear across this impedance. The impedance acts as a voltage divider for the tunnel diode RF oscillator. As each pulse at the unijunction fires, a current pulse is drawn through the tunnel diode circuit with a consequential increase and decrease in RF oscillator frequency. The RF carrier has a continuous modulation at approximately 8 millicycles per second with a subcarrier modulation depth of 40 per cent. The range of transmission is approximately 6 to 12 inches. The significance of using unijunction transistors is their ability to generate very stable oscillations without complex compensating circuits. The typical drift of unijunctions is only about 1 per cent, due to a 10 per cent drop in battery voltage. This advantage is combined with excellent temperature frequency stability (less than 1 per cent in the 50 to 100” F temperature range). Also, the use of unijunctions provides for simplicity of oscillator components, since only solid state epitaxial force sensors and capacitors are required for the subcarrier oscillators. A major problem in the use of unijunctions is that they are manufactured as nonpassivated devices. In their manufactured packaged form, unijunctions are much too large to be used for the size of the transmitter desired. The manufacturer of unijunctions was not able to provide the unijunctions in a passivated form, or in any package except a half-size TO-5 container which is approximately .3 by .3 inch in size. This sized package was much too large for use in the transmitter. Because of the size problem, it was necessary to develop a method for assembling unijunctions in a flat pack (.05 by .250 by .250 inch) as seen in Fig. 3. The unijunctions are sealed in dry nitrogen and tested for stability and voltage characteristics. During a 1,000 hour life-test study, a frequency change of less than 2 per cent was observed. At the completion of the life test, the frequency change from the original was only .06 per cent.

SIX-CHANNEL

INTRAORAL

TRANSMITTER

59

3 3 P

SCOTT AND

60

Fig. 3 .-A photograph sealing with dry nitrogen

ASH

J. Pros. Den. January-February, 1966

of the flat pack showing the unijunctions in position Millimeter markings appear at the left.

(left)

and after

(tight).

The present size of the transmitter package is .4 by .3 by .3 inches. Additional work in thin film deposits is being undertaken to reduce the size of the transmitters further. Although the replaceable Mallory RS-312 batteries can be used in many instances, the Bionetics TT-1003 batteries (.125 by .250 inch) are smaller and can be used, but only for short periods of time before having to be recharged. The force-detecting elements are solid state force sensors that require only a molecular physical change to exhibit a resistance change. They are bonded onto a metal form to detect the vertical and lateral forces encountered in occlusion. The sensitivity of applied forces can be detected in a range of approximately 0 to 100 pounds, depending on the receiver gain. RECEIVING

AND DECODING

EQUIPMENT

The receiver is designed to receive a 3 to 10 millicycle RF carrier. It has a band width of 200 kilocycles per second to pass all the subcarrier frequencies, and at the same time attentuate rapidly outside this band. Modulation can be AM or FM The output of the telemetry receiver is the six subcarrier frequencies. These six subcarrier frequencies are fed into six discriminators where they are mixed to 455 kilocycles per second in a balanced mixer. The signal is then amplified and limited in a narrow band IF amplifier and detected by a 455 kilocycle IF discriminator. The DC voltage signal from the discriminator is amplified and used to drive mirror galvanometers in the recording system. At present six channels of data on occlusal forces and six channels of electromyographic data are recorded on a 24 channel Honeywell Visacorder.

Volume Number

16 1

SIX-CHANNEL

Fig. 4.- The transmitter precision attachment.

INTRAORAL

TRANSMITTER

incorporated in a maxillary

61

molar pontic with fixed-removable

The transmitter is shown in the pontic of a precision attachment bridge which is capable of being fixed by small screws (Fig. 4). Although some developmental work on the transmitter is still in progress for further miniaturization, the present type of transmitter affords the opportunity for istudying occlusal forces, contacts, and other phenomena when fixed or removable partial dentures are used as carriers. The transmitter may be easily modified to transmit one to six channels of information. REFERENCES

1. Jankelson, B., Hoffman, G. M., and Hendron, J. A.: The Physiology of the Stomatognathic System, J.A.D.A. 46:375-386, 1953. 2. Jankelson, B. : Physiology of Human Dental Occlusion, J.A.D.A. 50:664-680, 1955. 3. Brewer, A. A., and Hudson, D. C.: Application of Miniaturized Electrotuc Devices to the Study of Tooth Contact in Complete Dentures. J. PROS. DEN. 11:62-72, 1961. 4. Neill. D. J.: A Study of the Contact Between Artificial Dentures Using Incorporated Radio Tra msmitters, Internat. D. J. 14:25.5-259, 1964. 5, Gr.af, H., and Zander, H. A.: Tooth-Contact Patterns in Mastication, J. PROS. DEN. 13:1055-1066, 1963. 6. Gillings, B. R. D., Kohl, J. T., and Zander, H. A.: Contact Patterns Using Miniature Radio Transmitters. J. D. Res. 42:177, 1963. 7. Adams, S. H., and Zander, H. A.: Functional Tooth Contacts in Lateral and in Centric Occlusion. J.A.D.A. 69:465-473, 1964. U.VIVERSITY OF MICHIGAN SCHOOL. OF DENTISTRY ANN ARBOR, MICH. 48104